Aluminum is conventionally produced commercially by the electrolytic conversion of alumina. The alumina is produced by well known procedures from aluminum hydroxide which is typically obtained by treating an aluminum-containing ore according to the Bayer process.
In the Bayer process, well known and conventional, an aluminum containing ore such as bauxite is digested with an aqueous alkali solution, usually an aqueous solution of sodium hydroxide, at elevated temperature to extract the alumina present. The resulting slurry contains soluble alkali aluminates and various materials insoluble in the alkaline solution. The slurry is then cooled, often by flash cooling and then filtered (clarified) to remove the insoluble solids. The supernatant alkali aluminate solution is then further cooled and seeded with aluminum hydroxide crystals to precipitate aluminum hydroxide. The precipitate is separated, washed, dried and then calcined at elevated temperature to produce the desired alumina. The alkali aluminate liquor of reduced aluminum content is recycled to the bauxite digestion step after removal of any excess accumulated water and provision of necessary make-up sodium hydroxide.
The alumina in most aluminum-containing ores is in the form of an alumina hydrate. In ores termed "bauxite", the alumina is generally present as a trihydrate, i.e., Al.sub.2 O.sub.3.3H.sub.2 O or Al(OH).sub.3, or as a monohydrate, i.e., Al.sub.2 O.sub.3.H.sub.2 O or AlO(OH). The trihydrate, termed "gibbsite", dissolves or digests more readily in the aqueous alkali solution than the monohydrate, termed "boehmite". Thus, bauxite ores containing major proportions of gibbsite digest at lower temperatures and pressures than do bauxite ores containing major proportions of boehmite.
In addition to the alumina hydrates, bauxite ores contain lesser amounts of silica (quartz), clay minerals such as kaolinite, iron oxides such as geothite or haematite, metal sulfides and organic matter including cellulose and humus. Depending upon the particular ore and the geological history of the site from which the ore was obtained, the type and proportion of these impurities will vary. Of particular concern is the proportion of organic matter present. A substantial part of the organic matter, e.g., up to 50% by weight or more, dissolves in the aqueous alkali aluminate solution during digestion. The presence of this soluble organic matter causes difficulties in the clarification and precipitation steps. The presence of the soluble organics lead to decreased efficiency in the aluminum hydroxide precipitation and certain types of organic matter form viscous liquors which reduce pumping and settling efficiency. Moreover, digestion of the organic matter consumes alkali which would otherwise be recycled.
It is known that these difficulties can be reduced if part or all of the organic matter is removed from the bauxite prior to the digestion step of the Bayer process. One known method of reducing the organic matter content of the bauxite is to calcine the ore prior to digestion. Considerable care must be taken during any type of heating bauxite ores to minimize the decomposition of any gibbsite present in the ore to boehmite. Such decomposition would decrease the efficiency of the subsequent digestion step.
In British Patent 1,383,136, a process is disclosed which involves calcination of gibbsitic bauxite containing ferric oxide. Substantial measures must be taken to prepare the ore in very small particles, for example, particles of a 10 .mu.m size, and the water content must be carefully controlled. The overall process is efficient, in part because of the presence of 10% by weight to 25% by weight of ferric oxide which helps prevent the transition of gibbsite to boehmite. However, the process is difficult because of the small size of the bauxite particles. In addition, the particular process conditions do not preclude decreased extractability of the resulting calcined bauxite. In European Patent Application No. 375987 there is described a process of calcining gibbsite bauxite wherein about 70% by weight of the organic matter, particularly oxalates, is removed. This process also uses rather small particles on the order of 63 .mu.m, and has the further disadvantage of rather incomplete removal of organic matter. The resulting calcined bauxite still contains about 0.1% by weight of organic carbon. It would be of advantage to have an improved process for the calcination of bauxite wherein the efficiency of alumina extraction is not reduced and a higher proportion of the organic matter is removed.